Conventionally, in a wet etching method for a workpiece such as, for example, a semiconductor wafer, an etching processing has been performed on a nitride film-oxide film by using phosphoric acid as an etching liquid. For example, an etching processing is performed on a workpiece such as, for example, a semiconductor wafer (hereinafter, referred to as a “wafer”) by heating an etching liquid composed of, for example, a phosphoric acid aqueous solution (H3PO4), which is stored in a processing bath, to a predetermined temperature, for example, 160° C. to 180° C., and circularly supplying the etching liquid of the predetermined temperature through a circulation line connected to the processing bath, and a circulation pump, and a temperature controller interposed in the circulation line (see, e.g., Japanese Patent Publication No. 3-20895).
In the etching method, when the etching processing is performed repeatedly, the silicon (Si) component of the wafer is eluted into the etching liquid, and the silicon (Si) concentration in the etching liquid is increased. Thus, the etching processing may not be performed precisely on the semiconductor wafer. Therefore, conventionally, it has been necessary to completely replace the etching liquid in the processing bath regularly.
However, when the etching liquid in the processing bath is completely replaced with a new etching liquid, the silicon concentration in the etching liquid is temporarily reduced. However, the lowered silicon concentration in the etching liquid is not a good thing, and maintaining the silicon concentration within a determined constant range enables the etching processing to be performed precisely on the semiconductor wafer.
In addition, when the etching liquid in the processing bath is completely replaced regularly, it is necessary to perform a liquid adjustment such as, for example, an adjustment of a silicon concentration of the new etching liquid by, for example, seasoning whenever the etching liquid is replaced. In this case, the liquid adjustment becomes complicated, and the processing time is increased as well.